proc: remove useless check on symlink removal

proc symlinks always have valid ->data containing destination of symlink.  No
need to check it on removal -- proc_symlink() already done it.

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>